A highly enantioselective amino acid-catalyzed route to functionalized α-amino acids

Article abstract of DOI:10.1021/ja017270h

The development of syntheses providing enantiomerically pure α-amino acids has intrigued generations of chemists and been the subject of intense research. This report describes a general approach to functionalized α-amino acids based on catalytic asymmetr

Full text of DOI:10.1021/ja017270h

Published on Web 02/08/2002
A Highly Enantioselective Amino Acid-Catalyzed Route to Functionalized
r-Amino Acids
Armando Co´rdova, Wolfgang Notz, Guofu Zhong, Juan M. Betancort, and Carlos F. Barbas III*
The Skaggs Institute for Chemical Biology and the Department of Molecular Biology,
The Scripps Research Institute, 10550 North Torrey Pines Rd., La Jolla, California 92037
Received October 9, 2001
Scheme 1. Approaches to R-Amino Acids Based on
Carbon-Carbon Bond-Forming Reactions
The development of syntheses providing enantiomerically pure
R-amino acids has intrigued generations of chemists and been the
subject of intense research. This effort has provided a diversity of
methodologies,1 which not only allow for the stereoselective
construction of naturally occurring amino acids but also permit the
rational design of optically active nonproteinogenic amino acids.
These amino acids in particular have enjoyed increased popularity
mainly due to their implementation into nonscissile peptide mimics
and peptide isosteres.2
example, the reaction with 2-butanone as donor (Table 1, entry 2)
gave rise to R-amino acid 2 in 72% yield with an ee >99%.
Significantly, the predominant diastereomer was formed from the
regioselective nucleophilic attack of the more substituted R-carbon
atom of the ketone. Very similar results were obtained with
3-pentanone, cyclohexanone, and 5-hexen-2-one as donors, afford-
ing the corresponding Mannich adducts 3-5 with excellent regio-,
diastereo-, and enantioselectivities (Table 1, entries 3-5). Although
the yield of 3 is lower due to a decreased reaction rate as compared
to the other donors studied, it is noteworthy that, for the first time,
3-pentanone was employed successfully in these proline-catalyzed
reactions.9 We also investigated the donor properties of substituted
acetone derivatives and found that the reactions of both fluoroac-
etone and hydroxyacetone furnished the corresponding Mannich
products 6 and 7, respectively, with very high regioselectivity (Table
1, entries 6 and 7). With hydroxyacetone as donor, a single
diastereomer was formed with an ee of 99%.10
In light of these results and the operational simplicity of our
reactions, we screened a number of solvents that are environmen-
tally more friendly as well as more suitable for potential industrial
applications and were pleased to find that in the reaction with
acetone as donor (eq 1), 1a could be obtained in comparable yields
and enantioselectivities in all the solvents studied.11 Moreover,
reducing the amount of proline to 5 mol % or the amount of ketone
donor12 to 2 equiv did not result in any loss of yield and enantio-
selectivity. This constitutes a significant improvement as compared
to the reaction conditions for related aldol and Mannich-type
reactions reported previously.13 The absolute stereochemistry at the
newly formed R-stereocenter was determined to be (S) by synthesis
and comparison to known isopropyl ester 1b (Table 1, entry 1).5d
Thus, L-proline catalyzes the synthesis of functionalized L-amino
acids. This stereochemical outcome is in accord with our previously
proposed transition states.6e,7d The Mannich products can be readily
further modified (Scheme 2), which includes facile oxidative
removal of the PMP group14 and in situ derivatization as well as
the diastereoselective reduction of the keto group yielding lactone
10. NMR studies of 10 and 11 also confirmed the anticipated syn-
stereochemistry.15
Several strategically different approaches toward R-amino acids
involving a carbon-carbon bond-forming event are conceivable
(Scheme 1, paths a-c). A very attractive solution is provided
through the addition of cyanide to an imine (Strecker reaction, path
a), which affords, upon hydrolysis of the intermediate R-amino
nitrile, the corresponding R-amino acid.
While the classical Strecker reaction yields racemic products, a
variety of asymmetric versions of this landmark reaction have
recently been developed utilizing both stoichiometric and catalytic
amounts of chiral sources.3 Two other very versatile approaches
consist of the addition of electrophiles to glycine enolate derivatives
(path b),4 and conversely, of the addition of nucleophiles to
electrophilic glycine templates (path c),5 both of which have been
used successfully as a methodology to access unnatural amino acids.
Following path c, we reasoned that, according to our previously
established concept of amino acid-catalysis involving enamine
intermediates,6 such an enamine formed between a ketone and
proline might serve as nucleophile and add stereoselectively to
R-imino esters in a Mannich-type reaction.7 Herein, we disclose
an important extension of amino acid-catalyzed direct Mannich-
type reactions using an R-imino glyoxylate as acceptor, which
provides novel functionalized R-amino acids with excellent regio-,
diastereo-, and enantioselectivities.
In an initial experiment, N-PMP-protected R-imino ethyl gly-
oxylate8 (0.1 M) and L-proline (20 mol %) were subjected to our
previously established standard conditions and stirred in acetone/
DMSO (1:4) at room temperature. After 2 h, the imine was
consumed and the corresponding R-amino acid 1a was formed as
the only detectable product and isolated in 82% yield and 95% ee
(eq 1).
Encouraged by this result, we further explored the scope of this
transformation by reaction of various ketones with N-PMP-protected
R-imino ethyl glyoxylate under the same reaction conditions (Table
1). In all cases, we found that the reactions proceeded smoothly,
typically affording only one single product in high yield. For
In summary, proline-catalyzed Mannich-type reactions of N-
PMP-protected R-imino ethyl glyoxylate with a variety of unmodi-
fied ketones provide functionalized R-amino acids 1-7 in high
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1842 VOL. 124, NO. 9, 2002 J. AM. CHEM. SOC.
10.1021/ja017270h CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Products from the Proline-Catalyzed Mannich-Reaction
of Unmodified Ketones with N-PMP-Protected R-Imino Ethyl
Glyoxylate
environmentally friendly solvents under operationally simple reac-
tion conditions. In the laboratory, these reactions were readily
performed on a gram scale. Further studies addressing the scope
and applicability are currently under investigation and will be
disclosed in a full paper.
Acknowledgment. This study was supported in part by the NIH
(CA27489) and The Skaggs Institute for Chemical Biology.
Supporting Information Available: Complete analytical data for
all new compounds (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
References
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889. (c) Williams, R. M. Aldrichim. Acta 1992, 25, 11. (d) Duthaler, R.
O. Tetrahedron 1994, 50, 1539. (e) Williams, R. M. In AdVances in
Asymmetric Synthesis; Hassner, A., Ed.; JAI Press: Stamford, CT, 1995;
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(2) For recent reviews, see: (a) Cole, D. C. Tetrahedron 1994, 50, 9517. (b)
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EnantioselectiVe Synthesis of â-Amino Acids; Juaristi, E., Ed.; Wiley-
VCH: New York, 1997.
(3) Stoichiometric: (a) Enders, D.; Shilvock, J. P. Chem Soc. ReV. 2000, 29,
359. (b) Davis, F. A.; Kee, S.; Zhang, H.; Fanelli, D. L. J. Org. Chem.
2000, 65, 8704. (c) Kunz, H. In StereoselectiVe Synthesis; Helmchen, G.,
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York, 1995; E21b, p 1931. Catalytic: For an account comprising the
important work of Corey, Hoveyda, Jacobsen, Kobayashi, Lipton, and
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Nogami, H.; Kanai, M.; Shibasaki, M Tetrahedron Lett. 2001, 42, 279.
(4) (a) Seebach, D.; Hoffmann, M. Eur. J. Org. Chem. 1998, 1337. (b) Aoyagi,
Y.; Jain, R. P.; Williams, R. M. J. Am. Chem. Soc. 2001, 123, 3472 and
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Scho¨llkopf, U. In Topics in Current Chemistry; Boschke, F. L., Ed.;
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K. V.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2001, 40, 2992. (c)
Knudsen, K. R.; Risgaard, T.; Nishiwaki, N.; Gothelf, K. V.; Jørgensen,
K. A. J. Am. Chem. Soc. 2001, 123, 5843. (d) Hagiwara, E.; Fujii, A.;
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Sinclair, P. J.; Zhai, D.; Chen, D. J. Am. Chem. Soc. 1988, 110, 1547. (f)
For the strategically similar nitro-Mannich reaction, see also: Yamada,
K.; Harwood, S. J.; Gro¨ger, H.; Shibasaki, M. Angew. Chem., Int. Ed.
Engl. 1999, 38, 3504.
(6) (a) List, B.; Lerner, R. A.; Barbas, C. F., III J. Am. Chem. Soc. 2000,
122, 2395. (b) Sakthivel, K.; Notz, W.; Bui, T.; Barbas, C. F., III J. Am.
Chem. Soc. 2001, 123, 5260. (c) Bui, T.; Barbas, C. F., III Tetrahedron
Lett. 2000, 41, 6951. (d) Betancort, J. M.; Sakthivel, K.; Thayumanavan,
R.; Barbas, C. F., III Tetrahedron Lett. 2001, 42, 4441. (e) Notz, W.;
Sakthivel, K.; Bui, T.; Zhong, G.; Barbas, C. F., III Tetrahedron Lett.
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ReV. 1999, 99, 1069. (b) Hirabayashi, R.; Ogawa, C.; Sugiura, M.;
Kobayashi, S. J. Am. Chem. Soc. 2001, 123, 9493. (c) Ishitani, H.; Ueno,
M.; Kobayashi, S. J. Am. Chem. Soc. 2000, 122, 8180 and references
therein. (d) List, B. J. Am. Chem. Soc. 2000, 122, 9336.
(8) The imine was preformed and isolated prior to the reaction. For its
preparation, see ref 5a. We have also investigated the one-pot three-
component Mannich reaction. In this case, however, the reaction does
not proceed to completeness and considerable amounts of the correspond-
ing aldol product are formed. PMP ) p-methoxyphenyl.
(9) This donor gave no products in related aldol reactions. See ref 6b.
(10) Hydroxyacetone also furnished excellent stereoselectivities in related aldol
reactions. See ref 6b and the following: Notz, W.; List, B. J. Am. Chem.
Soc. 2000, 122, 7386.
a PMP ) p-methoxyphenyl. b Isolated yields of pure product after column
chromatography. In a typical experiment, N-PMP-protected R-imino ethyl
glyoxylate (0.5 mmol) was dissolved in anhydrous DMSO (4 mL), the
corresponding ketone donor (1 mL) was added followed by L-proline (20
mol %), and the mixture was stirred for 2-24 h at room temperature.
Following aqueous workup with half-saturated ammonium chloride solution
and extraction with ethyl acetate, the organic layer was dried (MgSO4),
filtered, and concentrated and the residue purified by column chroma-
tography (silica, hexanes/ethyl acetate mixtures) to afford the corresponding
Mannich addition product. c dr ) syn/anti as determined by NMR. d The
ee values of products 1-7 were determined by chiral-phase HPLC analysis.
e Acetone was used as solvent.
Scheme 2. Further Modifications of Mannich Product ent-4a
(11) Acetone: 86% yield, 99% ee. CHCl3: 84%, 98% ee. EtOAc: 65%,
98% ee. Toluene: 81%, 83% ee. THF: 79%, 97% ee. 1,4-Dioxane: 81%,
98% ee.
(12) For this purpose, the following donors were studied: acetone, hydroxy-
acetone, cyclohexanone, and 2-butanone.
a Reagents and conditions: (a) PhI(OAc)2, MeOH, 0 °C, 30 min, 1 h;
1 N HCl, 0 °C, 30 min; (b) aqueous Na2CO3, Ac2O (64%), or Boc2O (68%).
(c) L-Selectride, THF, -78 °C, 89%.
(13) Only DMSO and CHCl3 provided satisfactory results in related aldol and
Mannich-type reactions. See refs 6b,e and 7d. In these cases, 20 mol %
catalyst and 20 vol % ketone donor were typically employed.
(14) Porter, J. R.; Traverse, J. F.; Hoveyda, A. H.; Snapper, M. L. J. Am.
Chem. Soc. 2001, 123, 10409-10410. The dr and ee were not affected
by this protocol. Removal of the keto group and ester cleavage of 9 pro-
vides access to (R)-cyclohexylglycine, which was recently posted on
www.InnoCentive.com.
yields with excellent regio-, diastereo-, and enantioselectivities. In
the case of 2-5 and 7, two adjacent stereogenic centers are created
simultaneously upon carbon-carbon bond-formation with complete
stereocontrol. Furthermore, the keto-functionality present in these
products offers a particularly attractive site for versatile modifica-
tions. Our methodology utilizes achiral, readily available, and rather
inexpensive starting materials, does not require any preactivation
of substrates or metal ion assistance, and can be carried out in
(15) See Supporting Information. Similar syn-selectivities in Mannich-type
reactions have been reported earlier. See refs 5a and 7b,d.
JA017270H
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